The present invention relates to a solid electrolyte making use of a microporous film of polymeric material. This microporous film may be used as a solid electrolyte material for batteries or as a solid electrolyte material for electric double-layer capacitors, and may be applied to an electrochemical device making use of electrochemical reactions as well. By the term xe2x80x9celectrochemical devicexe2x80x9d used herein is intended a device harnessing electrochemical reactions.
Electrochemical devices such as batteries, electric double-layer capacitors and electrochromic display devices are known as devices harnessing electrochemical reactions.
Various forms of batteries are now widely used in fields from electronics to automobiles, or they are provided in a large form for power storage purposes. In such batteries, electrolysis solutions are ordinarily used for electrolytes. Batteries using solid electrolytes instead of liquid electrolytes, on the other hand, attract attention as next-generation batteries, because leakage problems can be eliminated or a sheet form of batteries can be achieved.
Especially if lithium ion secondary batteries, etc. now increasingly used in the field of portable equipment such as notebook PCs are available in a sheet form or a slimmed-down laminate form, then their drastic application and development are expectable.
Various materials such as ceramic materials, polymeric materials and composite materials using both materials are put forward for such solid electrolytes. In particular, gel electrolytes obtained by plasticizing polymeric electrolytes using an electrolysis solution are thought of as holding great promise in view of the development of electrolytes, because they combine the high conductivity of a solution system with the flexibility of a polymeric material. One of the merits of batteries using such polymeric electrolytes is that they can be slimmed down with a large area or, in another parlance, can be used in a sheet form. This will spur the application and development of batteries. Examples of application of such gelled electrolyte materials to battery materials reported so far in the art have already been disclosed in U.S. Pat. No. 3,989,540. In more recent years, batteries making use of plasticized PVDF type copolymers have been known typically from U.S. Pat. Nos. 5,296,318 and 5,418,091. In particular, the batteries disclosed in U.S. Pat. Nos. 5,296,318 and 5,418,091 are found to be superior in discharge performance to conventional batteries using gel electrolytes, and batteries having very excellent rate performance are available as shown in U.S. Pat. No. 5,540,741.
However, the material system set forth in U.S. Pat. Nos. 5,296,318, 5,418,091 and 5,540,741 has a production problem although it has good rate performance. In other words, the PVDF type copolymer used in this system is more deformable, and lower in strength, than a homopolymer of the same PVDF type, because the copolymer is more easily swollen with an electrolysis solution and because of copolymerization. This is a problem resulting from the copolymerization of PVDF. The problem is due to low crystallizability, and is believed to be derived from the inherent nature of the material. As a result, a practically usable thickness must be at least 60 xcexcm. Thus, there is no denying that this material system is unfavorable when compared with the fact that the separator used with a conventional lithium ion battery using a solution is usually of the order of 25 xcexcm in thickness. As the capacity of a battery increases, the thickness of the battery becomes a graver problem than that of a solution type battery. Thus, it is still impossible to take full advantage of the battery using a gel electrolyte in reducing the thickness of the battery.
Another problem with the aforesaid material system is that the PVDF copolymer becomes locally thin due to the encroachment on the gel electrolyte of an expanded metal used as a collector, resulting possibly in a short circuit that is a grave obstacle to battery production.
In addition, it has been found that when the PVDF type copolymer impregnated with an electrolysis solution is stored at high temperatures, the electrolysis solution is released from the copolymer, unlike the homopolymer.
In short, the problems associated with the gelled electrolyte using the PVDF type copolymer are that:
1. its strength is low,
2. it is very difficult to fabricate an electrolyte having a thickness of 60 xcexcm or less,
3. a short circuit is likely to occur, and
4. it is poor in high-temperature performance.
One object of the present invention is to provide a solid electrolyte using a microporous film of high crystallizability and high strength. Another object of the invention is to use this solid electrolyte, thereby achieving an electrochemical device, a lithium ion secondary battery and an electric double-layer capacitor which can be further slimmed down and reduced in terms of the occurrence of short circuits with improved battery performance.
Yet another object of the present invention is to achieve an electrochemical device, a lithium ion secondary battery and an electric double-layer capacitor comprising a solid electrolyte using as an electrolyte film a microporous film having a suitable porosity without recourse to any plasticizer or the like.
In view of the aforesaid problems, the inventors have intensive studies to find out PVDF type a homopolymer material system that can eliminate the demerits of the PVDF type copolymer and make up for the demerits of the PVDF homopolymer.
As a result of studies made on the following three points, the inventors have now found that a solid electrolyte having functions equivalent to those set forth in the aforesaid prior art publications can be obtained by making improvements therein:
(1) a sheet form of film comprising a ceramic material,
(2) a film with PVDF homopolymer particles dispersed therein, and
(3) an improvement in the adhesion of a PVDF homopolymer microporous film to an electrode.
According to the present invention, an excellent solid electrolyte can be obtained through studies made on the aforesaid point (3). The microporous film in the solid electrolyte obtained according to the invention is quite different in structure from known film materials, because such plasticizers as shown in the prior art publications are not used at all.
That is to say, it is found as a result of close studies on the gel electrolytes known so far in the art that, in principle, they are generally broken down into the following three types:
1. An electrolyte film obtained by the swelling of chemical gel or physical gel.
2. An electrolyte film wherein a microporous film is simultaneously formed by a plasticizer. This amounts to a structure wherein micropores are present in the gelled film according to (1) above and both the gelled film and the microporous film function as an electrolyte. This electrolyte film corresponds to the solid electrolyte using the plasticizer disclosed in the aforesaid known examples.
3. A microporous film. This is characterized in that micropores exist in a three-dimensional manner, and the film material can be swollen with an electrolysis solution to retain the solution therein. These micropores are larger in pore diameter than those in the aforesaid electrolyte film (2).
The aforesaid object is achievable by the following embodiments.
(1) A solid electrolyte comprising a microporous film having high crystallizability and excellent solvent resistance, wherein:
said microporous film is controlled by a wet phase inversion method to a porosity of 50% or greater and a pore diameter of 0.02 xcexcm to 2 xcexcm inclusive.
(2) The solid electrolyte according to (1) above, wherein said wet phase inversion method allows a film-forming raw solution to be solidified in a solution obtained by mixing a phase-inverting organic solvent and water at a weight ratio of 100:0 to 60:40.
(3) The solid electrolyte according to (1) or (2) above, wherein said microporous film is formed of a material having a melt viscosity of 1,500 Paxc2x7sxe2x88x921 (230xc2x0 C., 100 sxe2x88x921) or greater.
(4) The solid electrolyte according to any one of (1) to (3) above, wherein said microporous film is formed of a material having a melting point of 150xc2x0 C. or greater and a heat of fusion of 30 J/g or greater.
(5) The solid electrolyte according to any one of (1) to (4) above, wherein said microporous film is formed of a poly(vinylidene fluoride) homopolymer.
(6) An electrochemical device comprising a solid electrolyte as recited in any one of (1) to (5) above.
(7) The electrochemical device according to (6) above, which is a lithium ion secondary battery.
(8) The electrochemical device according to (6) above, which is an electric double-layer capacitor.
(9) A solid electrolyte production process, wherein a solution of a raw material dispersed and dissolved in an organic solvent is solidified in a solution obtained by mixing a phase-inverting organic solvent and water at a weight ratio of 100:0 to 60:40 to obtain a microporous film having a porosity of 50% or greater and a pore diameter of 0.02 xcexcm to 2 xcexcm inclusive.
(10) The solid electrolyte production process according to (9) above, wherein the raw material for said microporous film is a poly(vinylidene fluoride) homopolymer.